The Border Gateway Protocol (BGP) is a common networking protocol that is used to back the core routing decisions in many networks, such as the Internet. Through BGP, routers may track network reachability among autonomous systems (AS), such as the separate networks or sub-networks that make up a larger network. BGP is a path vector protocol that does not use traditional Interior Gateway Protocol (IGP) metrics, but makes routing decisions based on path, network policies and/or rulesets. For this reason, BGP effectively acts more as a reachability protocol rather than a routing protocol.
Under BGP, network devices, such as routers, maintain a routing table that keeps track of paths (also called routes) between autonomous systems. For some networks, the routing table can be relatively large, which can be problematic if the physical resources, such as the available memory, of the network device are not able to store the entire routing table.